The present invention is directed generally to reflow soldering ovens and more particularly to reflow soldering oven flux collection systems and methods for removing flux from the oven atmosphere.
Reflow soldering ovens are used in the production of printed circuit boards employing surface mount technology. A reflow soldering oven is described in U.S. Pat. No. 5,993,500 entitled xe2x80x9cFlux management system,xe2x80x9d which is herein incorporated by reference. Typically, in a reflow soldering oven the products to be soldered pass through heating zones to a cooling zone. The heating zones are separated into a number of different zones which are generally broken down into preheat zones, soak zones and spike zones. In the preheat zones and the soak zones the products are heated and the flux volatile components vaporize in the surrounding gas atmosphere. The spike zones are hotter than the preheat zones and soak zones, and it is in the spike zones that the solder melts. A reflow solder oven may have many heating zones and these heating zones can be varied depending on the products to be soldered. Different products require different heat profiles and a solder oven should be flexible so that, for example, a machine with ten heating zones may have one preheat zone followed by seven soak zones and two spike zones for one type of circuit board, and a machine may have three preheat zones, six soak zones and one spike zone for a different type of board. The cooling zone or zones follow the heating zones and are used to solidify the solder on the board.
During production, a paste containing solder particles mixed with flux, adhesives, binders, and other components is applied to selected areas of a printed circuit board. Electronic components are pressed against the applied solder paste, while adhesives in the paste hold the components to the printed circuit board. A conveyor belt within a reflow oven carries the printed circuit board and components through a high temperature region within the oven where they are heated to a temperature sufficient to cause the solder particles in the paste to melt. Molten solder wets metal contacts on the components and printed circuit board. The flux in the solder paste reacts with the contacts to remove oxides and to enhance wetting. The conveyor belt moves the heated printed circuit board to a cooling region of the oven where the molten solder solidifies forming a completed electronic circuit.
The reaction of the flux with the contacts liberates vapors. Further, heat within the oven vaporizes un-reacted flux as well as the adhesives, binders, and other components of the solder paste. The vapors from these materials accumulate within the oven leading to a number of problems. If the vapors migrate to the cooling region they can condense on the circuit boards, contaminating the boards and making subsequent cleaning steps necessary. The vapors will also condense on cooler surfaces within the oven, clogging gas orifices, gumming up moving parts, and creating a fire hazard. This condensation may also drip onto circuit boards destroying them, or making subsequent cleaning steps necessary. In addition, condensed vapors may contain corrosive and toxic chemicals which can damage equipment and create a hazard to personnel.
The vapors generated by the reflow operation collectively are referred to herein as xe2x80x9cflux vapors.xe2x80x9d It is understood that the flux vapors can include vaporized flux, vapors from other components of the solder paste, reaction products released when the flux is heated, as well as vapors out-gassed from the printed circuit board and electronic components.
The use of flux management systems for filtering gas containing flux vapors from a reflow oven atmosphere is known. The most common method used for managing flux is condensing the gas containing flux vapors by blowing the gas from the reflow soldering oven across a radiator-type coiled or serpentine heat exchanger external to the oven. The fluid within the heat exchanger is typically room temperature air, chilled water or tap water. This method keeps the cooling fluid separated from the gas. The cleaned gas is returned to the oven.
Another method that is used is to recycle a portion of the gas through a heat exchanger to cool the gas down. The cooled gas is then used to bombard hot gas causing the flux vapors to condense and get trapped in an external filter.
Another method involves passing the gas through a series of external centrifugal blowers. The centrifugal force exerted on gas particles such as vaporized flux causes them to be forced radially outward and condense and drop out when they collide with an outer wall.
Finally, another method passes the gas through an external container with an oil bath and an assortment of filtration medium, which condenses and collects flux particles and returns the filtered gas back to the oven.
Each of these methods suffers from certain problems. Flux gases can contain a mixture of many components with a range of condensation temperatures, viscosities, and degrees of crystallization or polymerization, which can make it difficult to effectively solidify the components. Further, the mixture of flux gas components can vary depending on what type of solder paste is used for a particular assembly.
Cleaning of a heat exchanger is difficult given the nature of condensed flux vapors. Condensed flux vapors are generally not water-soluble so that a solvent is required to remove them. Many solvents are toxic and/or flammable, presenting a safety hazard to workers. Further, disposal of solvent waste is expensive, particularly when the solvent waste includes a variety of unknown reaction products and other chemicals from the condensed flux vapors.
It is therefore desirable to provide a flux management system that removes flux from the system while preventing or minimizing flux from dripping on the printed product. It is also desirable to provide a flux management system that can be cleaned with little or minimized maintenance downtime for the associated reflow oven.
In one embodiment of the present invention, a reflow solder oven having a cooling chamber for removing an amount of contaminants from a supply of gas is provided. In this embodiment, the cooling chamber includes an intake port for receiving the supply of gas, a heat exchanger containing a cooling medium in thermal communication with the gas to cool the gas; a collection pan for collecting contaminants formed when the heat exchanger cools the gas; and a drain operatively connected to the collection pan to drain the contaminants.
In another embodiment of the present invention, a filtering apparatus for a reflow oven is provided. The apparatus includes a cooling chamber to cool an amount of gas such that an amount of flux condenses out of the gas, a collection pan for collecting the amount of flux that condenses out of the gas, and a heater in thermal communication with the flux to heat and decrease the viscosity of the flux such that the cooling chamber is cleaned.
In yet another embodiment of the present invention, a filtering apparatus for a reflow oven is provided that includes a chamber for removing an amount of flux and other contaminants from a supply of gas; and self-cleaning means for removing the amount of flux and or other contaminants from the reflow oven.
In still yet another embodiment of the present invention, a method for removing an amount of flux from a supply of gas is provided. The method includes the steps of passing the supply of gas through a cooling chamber, wherein the amount of flux is condensed out of the gas, collecting the amount of condensed flux in a collection container, and introducing a heated supply of gas to interact with the gas and decrease the viscosity thereof.
An advantage of embodiments of the invention include being able to remove contaminants from the reflow oven without having to stop the operation of the oven, which increases the efficiency and maintenance costs of the reflow oven.